The term avermectin (previously referred to as C-076) is used to describe a series of compounds isolated from the fermentation broth of an avermectin producing strain of Streptomyces avermitilis and derivatives thereof. The morphological characteristics of the culture are completely described in U.S. Pat. No. 4,310,519. The natural avermectin compounds are a series of macrolides, each of which is substituted therein at the 13-position with a 4-(.alpha.-L-oleandrosyl)-.alpha.-L-oleandrose group. The preparation and properties of synthetic avermectin aglycones in which the disaccharide moiety has been removed leaving a free hydroxyl group at position 13 have been described by Mrozik el al., J. Org. Chem. 1982, 47,489-492 and by Chabala et al., J. Med. Chem. 1980, 23, 1134-1136. Additionally, U.S. Pat. No. 4,199,569 reveals the 22,23-dihydro avermectin compounds. The avermectin compounds and the instant derivatives thereof have a very high degree of anthelmintic and anti-parasitic activity. The natural compounds have the following general structure: ##STR1## wherein the broken line at the 22,23-position indicates a single or double bond and;
R.sub.1 is hydroxy and is present only when said broken line indicates a single bond; PA1 R.sub.2 is isopropyl or sec-butyl; and PA1 R.sub.3 is methoxy or hydroxy.
There are eight major natural avermectin compounds, designated A1a, A1b, A2a, A2b, B1a, B1b, B2a and B2b. These designations are based on the structure of the individual compounds as shown in the following table (referring to the foregoing structural formula).
______________________________________ Compound 22,23-bond R.sub.1 R.sub.2 R.sub.3 ______________________________________ A1a double bond -- sec-butyl --OCH.sub.3 A1b double bond -- isopropyl --OCH.sub.3 A2a single bond --OH sec-butyl --OCH.sub.3 A2b single bond --OH isopropyl --OCH.sub.3 B1a double bond -- sec-butyl --OH B1b double bond -- isopropyl --OH B2a single bond --OH sec-butyl --OH B2b single bond --OH isopropyl --OH ______________________________________
The avermectins are generally isolated as mixtures of the "a" and "b" components (typically .gtoreq.80% "a" and .ltoreq.20% "b"). Such compounds differ only in the nature of the R.sub.2 substituent and this minor structural difference has been found to have very little effect on the chemical reactivity or hie, logical activity of the compounds. Thus, although the "a" and "b" components can be separated from each other by chromatography this is not necessary and hence is not normally done. The presence of a mixture of "a" and "b" components may be indicated by dropping the "a" or "b" from the designation of the compound. A mixture of avermectin B1a and avermectin B1b is thus referred to as avermectin B1. Alternatively, a slash(/) is inserted between the compound designations to indicate a mixture such as in "B1a/B1b".
The above structural formula is shown without a definitive stereochemistry at certain positions and with a defined stereochemistry at other positions. However, during the course of the synthetic procedures used to prepare such compounds, or using racemization or epimerization procedures known to those skilled in the art, the products of such procedures can be a mixture of stereoisomers. In particular, the stereoisomers at the 13- and 23-positions may be oriented either .alpha.- or .beta.- representing such groups being below or above the general plane of the molecule, respectively. In each such case, and at other positions in the molecule, both the .alpha.- and .beta.- configurations are intended to be included within the ambit of this invention.
A related family of natural products is known as the milbemycins. The milbemycins have the same macrocyclic ring structure as the avermectins but have no substitution at position 13 and have a methyl or ethyl group at position 25 (R.sub.2 =methyl or ethyl rather than isopropyl or sec-butyl as in the avermectins). The milbemycins and the fermentation conditions used to prepare them are described in U.S. Pat. No. 3,950,360. Closely related 13-deoxyavermectin aglycones are prepared by chemical modification of the natural avermectins and have been described in U.S. Pat. Nos. 4,171,134 and 4,173,571.
Stabilization of the avermectin class of compounds depends on the particular compound of interest and the method of stabilization. For example, some avermectin compounds require the addition of antioxidants such as propyl gallate, BHA (butylated hydroxy anisole), BHT (butylated hydroxy toluene), monothioglycerol, vitamin E and the like, to the bulk product to inhibit degradation. Other avermectins have been stabilized by the formation of benzoate salts. The present invention is different in that stabilization of avermectins is significantly increased by recrystallization of the product with a sterically encumbered alcohol which results in a new form of avermectin molecule whereby the spatial arrangement of the alcohol in the crystal leads to enhanced thermal stability.